1. Field of the Invention
The invention is related to the field of magnetic recording heads and, in particular, to using a sacrificial layer that protects hard bias magnets during a chemical mechanical polishing (CMP) process.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more magnetic recording heads (sometimes referred to as sliders) that include read elements and write elements. An actuator/suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to fly a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves the actuator/suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
To read data from the magnetic disk, transitions on a track of the magnetic disk emit magnetic fields. As the read element passes over the transitions, the magnetic fields of the transitions modulate the resistance of the read element. The change in resistance of the read element is detected by passing a sense current through the read element, and then measuring the change in bias voltage across the read element to generate a read signal. The resulting read signal is used to recover the data encoded on the track of the magnetic disk.
The most common types of read elements are magnetoresistive (MR) read elements. A typical MR read element includes a MR sensor fabricated between a pair of shields. The MR sensor may be a Giant MR (GMR) sensor, a Tunneling MR (TMR) sensor, or another type of MR sensor. A GMR sensor implementing two layers of ferromagnetic material (e.g., NiFe) separated by a layer of nonmagnetic material (e.g., Cu) is generally referred to as a spin valve (SV) sensor. A simple-pinned SV sensor generally includes an antiferromagnetic (AFM) pinning layer, a ferromagnetic pinned layer, a spacer layer, and a ferromagnetic free layer. The pinned layer has its magnetization typically fixed (pinned) by exchange coupling with the AFM pinning layer. The pinning layer generally fixes the magnetic moment of the pinned layer perpendicular to the ABS of the recording head. The magnetization of the free layer is not fixed and is free to rotate in response to the magnetic field from the magnetic disk. The magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS in response to positive and negative magnetic fields from the rotating magnetic disk. The free layer is separated from the pinned layer by the nonmagnetic spacer layer.
A TMR sensor comprises first and second ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin, that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the TMR sensor, the ferromagnetic pinned layer has its magnetic moment pinned, while ferromagnetic free layer has its magnetic moment free to rotate in response to an external magnetic field from the magnetic disk. When a sense current is applied, the resistance of the TMR sensor is a function of the tunneling current across the insulating layer between the ferromagnetic layers. The tunneling current flows perpendicularly through the tunnel barrier layer, and depends on the relative magnetization directions of the two ferromagnetic layers. A change of direction of magnetization of the free layer causes a change in resistance of the TMR, which is reflected in voltage across the TMR sensor.
Designers of read elements use different techniques to stabilize the magnetic moment of the free layer. Although the magnetic moment of the free layer is free to rotate upwardly or downwardly with respect to the ABS in response to positive and negative magnetic fields from the magnetic disk, it is important to longitudinally bias the free layer (biased parallel to the ABS and parallel to the major planes of the layers of the read element) to avoid unwanted movement or jitter of the magnetic moment of the free layer. Unwanted movement of the magnetic moment adds noise and unwanted frequencies to the signals read from the read element.
One method used to stabilize the magnetic moment of the free layer is to bias the free layer using first and second hard bias magnets that are adjacent to the sides of the MR sensor. Examples of hard bias magnets are CoPt or CoPtCr. The magnetic moments of the hard bias magnets stabilize the magnetic moment of the free layer of the MR sensor.
To fabricate a read element with hard bias magnets, MR material is deposited on a first shield, and a first photoresist is patterned on the MR material to define a stripe height of an MR sensor. An ion milling process is then performed to remove the portions of MR material exposed by the first photoresist, and refill material is deposited. The first photoresist is then removed. Next, a chemical mechanical polishing (CMP) stop layer is deposited on the top surface of the MR material and the refill material. The CMP stop layer may be a diamond-like carbon (DLC) or another type of material. A bottom anti-reflective coating (BARC) layer is then deposited on the CMP stop layer, and a second photoresist is patterned on the BARC layer. The second photoresist is used to define the track width of the MR sensor. A reactive ion etching (RIE) process is then performed to remove the BARC layer and the CMP stop layer exposed by the second photoresist. An ion milling process is then performed to remove the portions of the MR material and the refill material exposed by the second photoresist. After the milling process, the stripe height and track width of the MR sensor is defined.
To form the hard bias magnets on either side of the MR sensor, a thin layer of insulation material is deposited. Next, hard bias material, including one or more seed layers and magnetic material, for the hard bias magnets is deposited. The hard bias material is typically deposited so that the top surface of the hard bias material is above the top surface of the CMP stop layer. A CMP process is then performed down to the CMP stop layer to planarize the top surface of the layers. The CMP process removes the second photoresist, and also removes the hard bias material that extends above the CMP stop layer. Thus, the CMP process defines the final thickness of the hard bias magnets. A second shield may then be deposited to form the read element.
When the thicknesses of the hard bias magnets on each side of the MR sensor are defined with the CMP process, the hard bias magnets on each side of the MR sensor may unfortunately have non-uniform thicknesses due to variations in the CMP process. The thickness variations may be between magnets on each side of the MR sensor, or between magnets on different read elements. Also, the top surfaces of the hard bias magnets on each side of the MR sensor may not be planar. When the hard bias magnets on each side of the MR sensor have non-uniform thicknesses and differently-shaped top surfaces, the hard bias magnets unfortunately have different effective magnetic fields. Thus, the hard bias magnets do not uniformly bias the magnetic moment of the free layer.
It would therefore be desirable to define the thickness of hard bias magnets in a different way.